The present invention generally relates to motor compressor units, and more specifically to motor compressor units employing motor caps and motor cap attenuation tubes to dissipate pressure pulses generated within the motor compressor unit.
The utilization of motor compressor units has become increasingly prevalent in recent years, particularly in refrigeration applications where the motor compressor unit is employed to compress refrigerant vapor. Conventionally, the motor compressor unit includes a compressor, a motor such as an electric motor, and a shell enclosing both the compressor and the motor. Typically, the compressor defines one or more cylinder chambers, a suction plenum which guides vapor to the cylinder chambers, and a discharge plenum which receives vapor from the cylinder chambers. A suction valve is positioned between the suction plenum and each cylinder chamber to control vapor flow therebetween, and a discharge valve is located between each cylinder chamber and the discharge plenum to regulate vapor flow therebetween. A reciprocable piston is movably disposed within each cylinder chamber, and the compressor also includes a rotatable crankshaft connected to both the motor and the compressor pistons.
In operation, low pressure vapor is drawn into the shell and through the motor to cool the motor. The vapor is then conducted into the compressor, specifically the suction plenum thereof. At the same time, the motor drives or rotates the compressor crankshaft, while rotation of the crankshaft reciprocates the pistons within the cylinder chambers. Reciprocating movement of the pistons within the cylinder chambers draws vapor thereinto from the suction plenum, compresses vapor within the cylinder chambers, and then discharges the compressed vapor into the discharge plenum. Therefrom the compressed vapor is conducted from the compressor and from the shell via a vapor discharge line. As is well understood in the art, the suction and discharge valves cyclically open and close to permit vapor flow from the suction plenum into the cylinder chambers and from the cylinder chambers into the discharge plenum.
This cyclic opening and closing of the suction valves, as is known by those skilled in the art, generates pressure pulses in the vapor flow path leading to the suction valves. These pressure pulses may be transmitted along the vapor flow path to the shell of the motor compressor unit, and the shell may transmit the pressure pulses to the ambient, producing undesirable noise. In order to dissipate these pressure pulses and to prevent the concomitant noise, a motor cap and a motor cap attenuation tube are often employed with motor compressor units of the general type described above. The motor cap is secured to and covers the top of the motor, and defines a small opening to allow vapor to flow into the motor cap and thence through the motor and to the compressor. The attenuation tube fits within the opening defined by the motor cap and extends outward therefrom to guide vapor into the motor cap. The motor cap and motor cap attenuation tube diffuse the pressure pulses developed by the suction valves, substantially reducing any noise generated thereby. Before the present invention, these attenuation tubes have been formed from a metal and then welded to the motor caps, which have also been made from metal. While these attenuation tubes perform very satisfactorily, they have a number of disadvantages; principally cost. The raw material for the tubes and the process of forming the tubes are both costly, and welding the attenuation tubes to the motor caps requires a considerable amount of time and skilled labor. Moreover, a metal attenuation tube, being a good conductor of heat, conducts heat from the motor cap to the vapor flowing through the attenuation tube, heating that vapor. This may have a slight adverse affect on the performance of the motor compressor unit. Further, the attenuation tubes previously used, in order to keep the manufacture of the tubes simple, have a uniform cross sectional shape, defining a vapor flow path of uniform cross sectional area. As vapor is drawn through this uniform area flow path, from the relatively large volume within the shell to the much smaller volume within the motor cap, some turbulence may develop in the attenuation tube. This turbulence may decrease the pressure of the vapor passing through the attenuation tube, also adversely affecting operation of the motor compressor unit.